An Improved Method for Estimating the Velocity Field of Coronal Propagating Disturbances

The solar corona is host to a continuous flow of propagating disturbances (PD). These are continuous and ubiquitous across broad regions of the corona, including the quiet Sun. The aim of this article is to present an improved, efficient method to create velocity vector field maps based on the direction and magnitude of the PD as observed in time series of extreme ultraviolet (EUV) images. The method presented here is for use with the Atmospheric Imaging Assembly (AIA)/Solar Dynamics Observatory (SDO) EUV channels and takes as input approximate to 2 hours of images at the highest 12 s cadence. Data from a region near disc centre is extracted, and a process called time normalisation is applied to the co-aligned data. Following noise reduction using a trous decomposition, the PD are effectively revealed. A modified Lucas-Kanade algorithm is then used to map the velocity field. The method described here runs comfortably on a desktop computer in a few minutes and offers an order of magnitude improvement in efficiency compared to a previous implementation. As applied to a region of the quiet Sun, we find that the velocity field describes a mosaic of cells of coherent outwardly-diverging PD flows of typical size 50 to 100 '' (36 to 72 Mm). The flows originate from points and narrow corridors in the cell centres and end in the narrow boundaries between cells. Visual comparison with ultraviolet AIA images shows that the flow sources are correlated with the bright photospheric supergranular network boundaries. Assuming that the PD follow the local magnetic field, the velocity flow field is a proxy for the plane-of-sky distribution of the coronal magnetic field, and therefore the maps offer a unique insight into the topology of the corona. These are particularly valuable for quiet Sun regions where the appearance of structures in EUV images is hard to interpret.